def isPrime(a):    
  if a == 1: return False    
  test = True    
  k = a - 1    
  while k > 1:    
      if not a % k:    
          test = False                
          break    
      k -= 1        
  return test   

n = input()  
r = []  
for i in range(len(n)):  
  for k in range(i + 1,len(n) + 1):  
      a = int(n[i:k]) 
      if isPrime(a):  
          r += [a]  
print(max(r) if r else 0)